Optically Enhanced Photon Recycling in Mechanically Stacked Multijunction Solar Cells

Multijunction solar cells can be fabricated by mechanically bonding together component cells that are grown separately. Here, we present four-junction four-terminal mechanical stacks composed of GaInP/GaAs tandems grown on GaAs substrates and GaInAsP/GaInAs tandems grown on InP substrates. The component cells were bonded together with a low-index transparent epoxy that acts as an angularly selective reflector to the GaAs bandedge luminescence, while simultaneously transmitting nearly all of the subbandgap light. As determined by electroluminescence measurements and optical modeling, the GaAs subcell demonstrates a higher internal radiative limit and, thus, higher subcell voltage, compared with GaAs subcells without the epoxy reflector. The best cells demonstrate $38.8 \pm 1.0\%$ efficiency under the global spectrum at $1000\ {\rm W}/{\rm m}^{2}$ and $\sim 42\%$ under the direct spectrum at $\sim$ 100 suns. Eliminating the series resistance is the key challenge for further improving the concentrator cells.